Method of preventing hydrate formation in petroleum well production strings



March 12, 1968 R. N. TUTTLE 3,372,753

NG HYDRATE FORMATION IN PETROLEUM METHOD OF PREVENT PRODUCING 20M? WELLPRODUCTION STRINGS Flled July 16, 1965 INVENTOR R.N. TUTTLE UnitedStates Patent 3,372,753 METHOD (3F PREVENTING HYDRATE FORMA- TIGN INPETROLEUM WELL PRODUCTIDN STRINGS Robert N. Tattle, Houston, Tern,assignor to Shell Oil Company, New York, N.Y., a corporation of DelawareFiled July 16, 1965, Ser. No. 472,598 3 Claims. (Cl. 166-45) ABSTRACT OFTHE DISCLGSURE The formation of hydrates and deposition of solids in aproduction tubing string carrying petroleum Well fluids having a highhydrogen sulfide, dissolved sulfur and water content may be prevented bymaintaining the fluid pressure within the production tubing throughoutthe length thereof above the bubble point of said well fluids.

duction techniques that as the produced fluid flows upwardly through thetubing string certain pressure-temperature conditions may occur suchthat a phenomenon known as fluid flashing results. The phenomenon offluid flashing is accompanied by a severe temperature drop which causesthe sudden formation of hydrates that plug the tubing string to theextent that production is sub stantially reduced or, in many cases,completely prevented.

In most wells the aforementioned hydrates form only at the lowertemperatures reached at the upper end of the tubing string.Consequently, in the past, the prevention of hydrate formation has beencontrolled by the application of heat to the upper end of the tubingstring thereby keeping the temperature above the critical level.However, hydrates occur at much higher temperatures in fluids containinghydrogen sulfide than in the production of normal hydrocarbon fluidsand, as a result, may plug the tubing string at great depths. Forexample, hydrates can occur at 120 F. in a fluid containing 80% hydrogensulfide. Accordingly, prior art methods, such as the application of heatto prevent hydrate plugging, are impractical and ineffective in theproduction of fluids containing substantial amounts of hydrogen sulfidesince there is no effective and economical way to transmit the amount ofheat required to the great depths at which such plugging occurs.

Moreover, the production of fluids containing substantial amounts ofhydrogen sulfide is additionally complicated by the deposition of solidsulfur throughout the length of the tubing string irrespective of theaforementioned hydrate problems associated with the production of thesefluids. As the fluid containing hydrogen sulfide flows upwardly throughthe tubing string the pressure on the fluid decreases due to thereduction of the static head in the fluid column. It has been found thatsulfur solubility decreases markedly with decreases in pressure andtemperature and as a result the sulfur deposits in the tubing string asthe fluid flows upwardly toward the surface. If not prevented, thesulfur deposits will accumulate and plug the tubing string.

Therefore, it is the primary object of the invention to produce a wellin a manner so as to prevent plugging of the production tubing string byeither the formation of hydrates and/or the deposition of solidmaterials out of the produced fluid.

Broadly, the invention comprises niaintaining the tubing string pressureat a value sumcient to prevent hydrate formation and/or soliddeposition. Of course, it is to be understood that this maintainedpressure value will vary depending upon the composition of theproduction fluid, etc.

More specifically, the invention comprises producing a fluid having ahigh hydrogen sulfide content (e.g. 50 H S or H 5 with dissolved sulfur,the balance being primarily hydrocarbons containing water or watervapor) by maintaining the pressure on the said fluid above the bubblepoint throughout the Well bore to prevent (l) hydrate formation (due tothe lower temperature which results from flashing etc.) and (2) sulfurdeposition resulting from the decreased solubility of sulfur atdecreased pressures and temperatures. One particular aspect of theinvention comprises the application of an external energy source, suchas a bottom hole pump or compressor, to maintain the pressure in theproduction tubing string at or above a pressure at which hydrates form,and/or solids deposit, as determined from thepressure-volume-temperature evaluations of the produced fluids.

Other objects and advantages or" the invention will be understood fromthe following detailed description taken with reference to theaccompanying schematic drawing.

Referring to the drawing, there is shown a well casing id extending froma location at the surface of the earth iii downwardly into a productionzone such as an oilbearing formation. The lower end of casing it) has acircumferential perforations 12 to allow fiiuds from the producing zoneto enter the casing 10. The casing 10 is closed at its upper end by acover member 13. A first tubing string 14 extends downwardly from thecasing cover member 13 and terminates near the lower end of the casing10 adjacent the producing zone. A conventional sealing packer E5 ispositioned in the lower end of the casing 10 to stabilize the lower endof the tubing string 1'4 which extends through a perforation formedtherein. The sealing packer 15 is preferably positioned at or near theupper strata of the producing zone to prevent fluid communicationbetween fluids being produced from the roducing zone and the interiorportion of the casing 10 located above the packer 15. A second tubingstring 16 extends downwardly from the casing cover member 13 to alocation slightly above the packer 15 Where it communicates, through abranched connection 17, with the interior of the first tubing string 14.

A fluid operated bottom hole pump Ztl is shown as seated on an annularinwardly protruding landing shoulder 21 formed within the tubing string1d. The outer diameter of the pump Zll is slightly smaller than theinner diameter of the tubing string 14 so that the pump may be readilylowered to the landing shoulder 21 and retrieved therefrom. To insurethat the produced fluids from the producing zone do not pass upwardlyaround the out-side of the pump 20 when the latter is seated on thelanding shoulder 21 of the tubing string 14, the pump is preferablyprovided with O-ring sealing members (not shown) about its circumferenceon both its upper and lower ends.

A power oil inlet opening 22 is provided on the upper end of the pump 20and a power oil outlet opening 23 is provided in the side of the pump.The outlet opening 23 communicates with a circumferential groove 24formed on the outer surface of the pump 20. The tubing string 14 isprovided with a bore 25 which is located a measured distance from theinternal landing shoulder 21 so that when the pump 20 is lowered intoposition on the landing shoulder the outlet groove 24 of the pump andthe bore 25 of the tubing string 14 will be laterally adjacent oneanother thereby allowing spent power oil to escape upwardly through theannular space 27 to the upper end of the casing 10. The lower end of thebottom hole pump 20 is provided with an inlet opening 29 which allowsproduced fluids from the producing zone to flow upwardly through theinlet opening 2/ and into the lower portion of the pump 20. An outletOpening 30, for the escape of produced fluids, is formed in the side ofthe pump 20 at a location adjacent the branched section 17 whichcommunicates the tubing string 14 with the tubing string 16. The outletopening 30 communicates with a circumferential groove 31 formed in theouter surface of the pump 20 to allow the produced fluids to flow out ofthe pump outlet 30 into the tubing string 16, regardless of whether ornot the outlet 30 is axially aligned with the branched section 17. Thepump 20 is a conventional fluid operated bottom hole pump whoseoperation will be more fully described infra.

The bottom hole hydraulic pump 20 is supplied with operating energy bymeans of a variable delivery pump 33 located on the surface 11. A poweroil storage reservoir 35 is preferably provided near the pump 33, andcommunicates therewith through a pipe 34, to insure that the drivingpump 33 may supply an adequate and uniform delivery of power driving oilto the bottom-hole pump 20.

As shown, a reduced diameter tubing 37 extends from the variabledelivery driving pump 33 down through the cover member 13 of casing toallow fluid communication between the tubing 37 and the first tubingstring 14. A second reduced diameter tubing 39 extends through thecasing cover member 13 to allow fluid communication between the casingannulus 27 and the power oil storage reservoir 35.

A third tubing 41 penetrates through the casing cover member 13 intocommunication with the second tubing string 16. A choke or pressurerelief valve 43 is located in the tubing 41 to insure that an adequatesurface back pressure (Le, a back pressure suflicient to prevent hydrateformation and/or solid deposition) will be maintained on the producedfluid flowing upwardly through the second tubing string 16. A pipe 45may lead to any desired facility for storing the produced fluids.

In the operation of the system the produced fluid will flow out of theproducing zone up through the end of the first tubing string 14 locatedbelow the packer and into the lower inlet opening 29 of the bottom-holepump 20. The pump is a standard conventional hydraulically operateddevice which is equipped with a standing valve (not shown) in the lowerinlet 29 and a movable piston (not shown) located in its upper end andadapted to move downwardly when power oil is supplied under pressurethrough the upper inlet opening 22. As the piston moves downwardly,under the pressure of the power oil, supplied from the driving pump 33,the standing valve in the lower inlet 29 closes and the produced fluidwhich has entered the lower end of the pump 20 is forced out of theoutlet opening into the second tubing string 16.

The pump 20 is designed in a conventional manner so that when the saidpiston reaches the bottom of its stroke the spent power oil flows intothe casing annulus 27 via the outlet port 23 and the bore 25 of the pumpand first tubing string 14, respectively, and then back to the storagereservoir via the tubing 39. The piston then returns to its uppermostposition and the standing valve in the lower pump inlet 29 opens toallow more fluid from the producing zone to enter the lower end of thepump 20. By adjusting the valve 43 an adequate surf-ace back pressuremay be maintained on the produced fluid to assure an operating pressurein excess of the bubble point pressure from the bottom-hole pump 20 tothe surface.

Preferably, the pressure on the produced fluid in the tubing string 16is maintained at a minimum of 200 p.s.i. above the bubble point so thatthe fluid will remain a saturated liquid thus preventing the formationof hydrates and/or the deposition of solids within the tubing string 16.This minimum pressure requirement of 50-200 p.s.i. above the bubblepoint is generally suflicient to account for the inherent fluctuationswhich occur in known pumping systems of the type herein disclosed. Forexample, the pressure is maintained at a minimum of 1500l700 p.s.i. fora fluid containing percent hydrogen sulfide.

In some instances it has been found that the bottomhole pressure at theproducing zone is sufficiently high to allow production of the fluidwithout the necessity of supplying an external source of energy. Underthese circumstances the bottom-hole pressure is suificiently high tomaintain the pressure of the produced fluids above the bubble pointthroughout the length of the tubing string.

Therefore, a modified form of the invention comprises extending a singletubing string from the surface down to the producing zone and employinga choke or pressure relief valve, such as shown at 43, in the singletubing string at the surface. By adjusting the choke valve, the pressurethroughout the tubing string may be maintained above the bubble pointthereby preventing hydrate formation and solid deposition without theuse of a bottomhole pumping system.

I claim as my invention:

1. In the production of an underground reservoir which produces a fluidcontaining hydrocarbons, moisture and over 50% hydrogen sulfide, amethod of preventing hydrate formation and sulfur deposition within atubing string extending from the surface down into communication withsaid reservoir fluid, said method com prising:

(a) extending a first tubing string within a well bore from the surfacedown into communication with said reservoir fluid;

(b) emplacing a sealing element circumferentially around the exterior ofsaid first tubing string and into sealing engagement with said well boreat a location near the upper extent of said reservoir;

(0) extending a second tubing string within the well bore from thesurface down into fluid communication with said first tubing string at alocation near the top of said sealing element;

(d) positioning a hydraullically actuated bottom-hole pump within saidfirst tubing string at a location such as to cause produced fluids fromsaid reservoir to flow upwardly only through said second tubing string;

(e) placing a valve means within said second tubing string at thesurface to regulate fluid flow through said second tu bing string; and,

(f) operating said bottom-hole pump and said valve means to maintain thepressure within said second tubing string above the bubble point of saidreservoir fluid.

2. A method as in claim 1 wherein said produced fluid contains 50-100%hydrogen sulfide.

3. A method as in claim 1 wherein said pressure is maintained at least50-200 p.s.i. above the bubble point of said produced fluids.

References Cited UNITED STATES PATENTS 371,006 10/1887 Waits. 3,223,15612/1965 Peter 166-45 X OTHER REFERENCES Uren, Petroleum ProductionEngineering, 1953, pp. 601-602.

Journal of Petroleum Technology, July 1960, pp. 166l69 incl.

ERNEST R. PURSER, Primary Examiner.

